Variable relief means for a hydraulic drive

ABSTRACT

VARIABLE RELIEF MEANS FOR A HYDRAULIC DRIVE ARE DISCLOSED FOR INCORPORATION INTO A HYDRAULIC DRIVE ARRANGEMENT TO CUSHION THE DRIVE FROM SUDDEN PRESSURE BUILDUPS AS WHEN THE DRIVEN INSTRUMENTALITY ENCOUNTERS IRREGULAR RESISTANCE. A CYLINDER RECEIVES A TRANSIENT FLOW CONSTITUTING A PORTION OF A HYDRAULIC PUMPS OUTPUT. A PISTON WITHIN THE CYLINDER HOUSES ORIFICES THROUGH WHICH FLUID ESCAPE IS METERED. IN COMMUNICATION WITH THE CYLINDER INTERIOR IS A RELIEF VALVE. A REMOTE PILOT VALVE CONTROLS THE RELIEVING PRESSURE OF THE RELIEF VALVE. LINKAGE INTERCONNECTS THE REMOTE PILOT VALVE AND THE PISTON WHEREBY UPON PISTON MOVEMENT RESULTING FROM A SUDDEN PRESSURE BUILDUP, THE PILOT VALVE WILL GRADUALLY INCREASE THE PRESSURE SETTING AT WHICH THE RELIEF VALVE WILL FUNCTION. ACCORDINGLY PRESSURE IS GRADUALLY INCREASED TO THE OPENING PRESSURE VALUE OF A MAIN RELIEF VALVE IN THE DRIVE SYSTEM FOR COMPLETE SYSTEM UNLOADING.

United States Patent Primary Examiner-M.-Cary Nelson AssistantExaminer-R. B. Rothman Attorney-James D. Givnan, Jr.

ABSTRACT: Variable relief means for a hydraulic drive are disclosed forincorporation into a hydraulic drive arrangement to cushion the drivefrom sudden pressure buildups as when the driven instrumentalityencounters irregular resistance. A cylinder receives a transient flowconstituting a portion of a hydraulic pumps output. A piston within thecylinder houses orifices through which fluid escape is metered. In communication with the cylinder interior is a relief valve. A remotepilot valve controls the relieving pressure of the relief valve. Linkageinterconnects the remote pilot v alve and the piston whereby upon pistonmovement resulting from a sudden pressure buildup, the pilot valve willgradually increase the pressure setting at which the relief valve willfunction. Accordingly pressure is gradually increased to the openingpressure value of a main relief valve in the drive system for completesystem unloading.

L lli VARIABLE RELIEF MEANS FOR A HYDRAULIC DRIVE BACKGROUND OF THEINVENTION The present invention relates generally to relief means forhydraulic systems and more particularly to a variable relief meansoperable at increasing pressure for use in a hydraulic system to permitgradual pressure increase in the system to avoid undesirable rapidpressure fluctuations.

It has been found highly useful to incorporate the present inventioninto ditching machines having, in an exemplary manner, a continuoustoothed digging chain operable as the machine slowly advances along itscourse. The machines engine jointly propels the vehicle andsimultaneously powers the digging chain both through drive components ofwhich at least some are in related and direct mechanical drive. Inoperation of the machine, the ground resistance or obstructions to thedigging chain will vary considerably from simply slowing the speed ofthe ditching machine, to actually stopping the machine. Suchencountering of trenching obstrudtions results in sudden loads beingexerted on the chains drive components which, in view of theirrelationship with the machines track driving components, results inrapid changes in the torque ultimately delivered to the track drivecomponents. Obviously this varying of torque to such components (thedifferential primarily) causes rapid part wear and early failure. Theuse of adjustable clutches in the digging chain drive has-failed toremedy the problem.

Additionally, in regard to wear of the machines drive components, thejerky forward movement characteristic of such machines results from thedigger chain teeth encountering obstructions preventing steady forwardmovement of the machines chain carrying bar. This results in rearwardjerking of the bar on the machine chassis causing erratic loading of thevehicle drive components.

The foregoing is by way of setting forth existing problems in the fieldof ditch digging equipment and in no way is it intended to implyrestriction of the application of the present invention to use with suchdigging equipment.

To remedy the foregoing problem of severe wear on the machines drivecomponents I have incorporated a hydraulic drive into the ditchingmachine to simultaneously power both the traction elements and thedigging chain. Associated with this hydraulic drive is the fluidcushioning means embodying the present invention. Other usefulenvironments for the invention will become apparent as the invention isfurther elaborated upon. Provision is made for receiving a quantity ofthe pumps output when the line pressure between pump and motor exceeds anormal operation pressure.

By way of example, when the digging chain encounters minor momentaryobstructions the resultant momentary loading of the hydraulic motorresults in a pressure increase in its fluid supply line which increaseis alleviated by temporary diversion of the fluid into and through achamber. Upon the digger bar and chain passing the ground obstructionthe supply line pressure returns to normal with entire pump output againdirected to said motor.

Important to the invention are means responsive to the rate of fluidpressure increase. When a sudden pressure buildup in the motor supplyline occurs as from stoppage of the digging teeth by a rock, a piston isdisplaced by the fluid. An adjustable pilot valve is operatively linkedwith the piston and upon piston movement past a settable point theoperating pressure of the pilot valve will progressively increase thusincreasing its associated relief valve cracking or venting pressure.Accordingly as the increasing pressure in the motor supply line causesrearward travel of the piston a transient volume of fluid is accumulatedin the expansible chamber. In most instances the increased pressure willultimately cause the digger tooth to overcome the obstacle, followed byreturn of the system to normal pressure. During this time the pilotvalve may operate to open the relief valve at a preset pressure valvesomewhat above normal operating pressure. In cases where the obstacle isnot so easily overcome the line pressure will build causing furtherpiston movement, at which time it will act via linkage on the pilotvalve to raise or increase the value at which the relief valve will ventor unload. By gradual movement of the piston, accomplished by orificesor ports therein, a greater operating pressure for the chain drivingmotor is gradually achieved without physical shock to the components.Accordingly the pressure buildup may progress at a relatively slow rateup to a main relief valve dumping pressure. For instance, assuming theobstacle is not overcome and the digging chain is momentarily stoppedthe gradually building pressure will cause the pilot valve to eventuallyclose off the venting port of the present relief valve at which timehydraulic pressure will build until a system main relief valve, having apreset maximum value, will unload the entire pump output to the systemtank.

This extreme example will be accomplished without fluid lock orresulting shock occurring to the hydraulic system. The hydraulic motorsspeed will of course vary with amount of oil being dumped, decelerationrate will be in relation to severity of obstruction and resultant pistonmovement.

BRIEF DESCRIPTION OF THE DRAWING In the accompanying drawing:

FIG. 1 is a plan view of the variable relief means with some componentssectioned for illustrative purposes,

FIG. 2 is a view taken along line 2-2 of FIG. 1 showing details of thelinkage for activating the pilot valve,

FIG. 3 is a view of a piston insert removed from its piston, and

FIG. 4 is a schematic of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS With continuing regard to thedrawing wherein applied reference numerals indicate parts similarlyidentified in the following specification, the reference numeral 10indicates a base plate for the mounting of components of the variablerelief means.

A cylinder 11, shown sectioned and defining a chamber 11A, is confinedintermediate a forward end plate 12 and a rear end plate 13interconnected adjacent their corners by four strain rods 14, therearward ends of which receive nut elements 15. The forward end plate 12receives a hose end fitting 16 to admit pump discharged fluid from apressure line 19 into an annular recess 17 in the end plate 12 while asimilar fitting l8 directs fluid toward one or more hydraulic motors notshown. Each end plate is suitably secured to the underlying base plate10. Centrally located within end plate 12 is a bushing 20 slidablyengaging one end of a piston rod 21. The bushing 20 incorporates O-Ringseals as at 22 and is secured to the plate 12 by machine screws.

Rear end plate 13 similarly mounts a like bushing at 24 retainingO-rings at 26. Each end plate is annularly recessed to receive theopposite ends of the cylinder 11 with a seal effected between the endsof the cylinder and the end plates by large diameter O-rings 27 and 28.The latter are confined within annular grooves machined on a radius tolocate the 0- rings 27-28 to receive the end surfaces of the cylinder.Further, the end plates 12-13 are shouldered at 12A-l3A to receiveO-rings 25.

A port is located on the interior surface of end plate 13 forcommunication with a fluid exhaust passageway at 30 for fluidcommunication with a relief valve at 31.

The piston rod 21 extends through the end plates 12 and 13 and isthreaded at 21A to receive a piston 33 having O-rings at 34. Said pistonand piston rod comprise a piston assembly. Providing fluid communicationfrom the pistons front side at 39 to the opposite or rear face 40 arepiston inserts 35 having fluid passageways 37 therethrough constitutingorifice means. Each orifice insert is further provided with a slot 38 topermit torquing of the insert into securement within partially threadedpiston bores. Accordingly the flow rate through the piston may be variedby substitution of other inserts having different diameter passageways37 to accommodate the particular installation requirements. A flange 41on the piston rod abuts against the piston to secure same in place.Extending inwardly from the rearward end of the piston rod is a bore 218of a diameter to permit telescopic advancement over a part of thelinkage for control ofa remote pilot valve indicated at 42.

A helical spring 43 is'concentrically disposed about the piston rod 21between the back face of the piston and the rear end plate 13 the springfunctioning to bias the piston and piston rod assembly to the forwardposition shown.

Afflxed by machine screws 45 to the back side of the rear end plate 13is a relief valve coupling 46. The relief valve 31 accordingly is influid communication with the chamber 11A defined by the cylinder 11.

The relief valve and its associated remote pilot valve 42 are incommunication via hydraulic hose segment .48. Suitable fittings as at 50on the relief valve and at 51 on the pilot valve communicate fluid tothe pilot valve through a restricted orifree in the relief valve. Theoperating relationship of a relief valve to a remote pilot valve isbelieved well known in the art and is summarized by saying that at aset, predetermined pressure value the pilot valve will allow venting ofthe associated relief valve. In effect a port in the relief valve isopened to allow fluid escape back to a reservoir. A reservoir returnline is indicated at 52. The mechanical structure description of the twovalves is not believed necessary as they are conventional. For practicalpurposes, relief and pilot valves manufactured by the Dennison Divisionof the Abex Corporation, series R2 and REO4 respectively, have beenfound suitable.

The following described pilot valve linkage is actuated by the pistonrod at a point in its rearward movement and functions to vary theloading on the pilot stem 53 (FIG. 4) the end of which protrudes fromthe pilot valve. Piston rod movement rearwardly beyond said pointeffects an increase in the loading of the pilot stem and consequently anincrease in the pressure value at which the relief valve will open. Thelinkage includes a control rod 54 threaded at 54A and axially alignedwith the piston rod 21. The diameter of rod 54 is ofa size to admittelescopic advancement of the bored end 218 of the rearwardly movingpiston rod 21. Contactable by the rearward end of the piston rod is acontrol rod spring 55 which upon being contacted resiliently impartsmovement to a control rod support member 56 within which the end of saidrod is secured. An adjustable and lockable nut 57 maybe advancedforwardly along rod 54 to position spring 55 to thereby determine thepoint of contact with the end ofpiston rod 21. In pinned engagement at58 (FIG. 2) with the member 56 is a connecting link 60 similarly securedat its forward end at 61 to permit slight lateral movement of the linkas later explained. A set screw 59 locks the nut 57 in its adjustedposition.

A pilot valve lever 63 is bifurcated at 64 to receive the end of thelink 60 and the pivot pin 61. The opposite end of lever 63 is alsobifurcated and is swingably mounted at 65 to a bracket 66 affixed to andpart of the pilot valve. The pilot stem 53 extends outwardly of thepilot valve body terminating in abutment with a pin 67 secured in thepilot valve lever and from this arrangement it will be apparent that thelever 63, upon rearward movement initiated by piston rod movement, theloading of the pilot stem 53 will increase. A tank return line at 69 isprovided for the pilot valve 42.

An adjusting screw 68 is for manually presetting the load on the pilotstem to determine the pressure value at which the relief valve 31 willstart venting to the reservoir. This pressure value will remain constantuntil piston assembly movement influences the rearward biasing of theabove described linkage.

The following operation of the present variable relief means is setforth in relationship to a digging machine, applications in otherhydraulic drive systems are believed apparent.

In conventional digging machines of the self-propelled type both frontand rear axles are powered each having a differential. Engine output isdelivered through a transmission, hydraulic pump and motor with thelatter delivering power via a roller chain and divider box to the twoseparate axle drive lines of the vehicle. The hydraulic motoradditionally drives, through V-belts, associated sprockets and clutchthe linked teeth ofa digging chain. The chain is entrained about a chainbar which bar is arcuately positionable within a vertical plane todetermine the digging depth.

From the above it will be seen that severe sudden loads may be impartedfrom the digging chain back through the vehicle drive lines to eachdifferential whereat failure most commonly occurs. The erraticapplication of such loads occurs primarily when the digging bar isoperating at a shallow angle relative to the grounds surface. Anyobstacle encountered by a tooth is translated into a rearward pull onthe digging bar and ultimately the chassis of the machine. When thedigging bar is more nearly in perpendicular relationship to the groundsurface such a force is primarily downward on the vehicle.

The existing arrangement of both vehicle and chain drive componentsabove set out is varied in the following manner. A positive displacementhydraulic pump is driven by the transmission output shaft. The pumpoutput is discharged past system main relief valve thence the presentinvention. A pair of directional valves receive the flow in pressureline 19, the first of which may direct the flow simultaneously to thedigging chain motor and the vehicle traction motor with the fluid to thelatter being regulated by a needle valve. The second directional valvedownstream from the first may direct the entire flow to the tractionmotor being rendered with the chain motor inoperative.

During normal trenching a flow is directed through the recess at 17 atthe forward end of the cylinder 11 with fluid at the same pressureoccupying the chamber area 11A rearward of the piston; the exhaustpassageway 30; the relief and pilot valves and their interconnectingline 48. A description is provided as follows for a slow building andsubsequently for a rapid pressure buildup.

When the digger teeth gradually encounter more resistance, the linepressure will increase resulting in a pressure increase in chamber 11A,passageway 30, relief and pilot valves and line 48. if the buildupcontinues the relief valve will eventually vent and unload to thereservoir. Valve 31 must vent before any flow through orifices canoccur. There is a pressure differential between cracking and full openpositions in the relief valve. The pressure at which this unloadingoccurs is determined by the manually preset adjusting screw 68. Theresulting escape of fluid from the area rearward of piston 33 incombination with the elevated line pressure results in a fluid flowthrough the orifices in the inserts 35. Under this condition when thepiston insert orifices cannot pass fluid at the same rate fluid is beingdumped by the relief valve the greater the pressure now exerted on thefront piston face 39 will cause rearward piston movement. Assumingpassage of the digger bar past the obstruction occurs before the end ofrod 21 contacts spring 55 the system will return to normal all occurringwith relatively rapid pressure fluctuations being averted. During thisfunction the partial rearward movement of the end of piston rod 21 hasnot occurred to the extent necessary to effect movement of spring 55 andthe pilot valve linkage from its static position.

In case of rapid increase in line pressure as by the digging teethencountering hard ground material that presents sustained resistance totooth passage, the pressure increase will eventually result in fullrearward movement of the piston and piston rod assembly. While somefluid escape will occur through the piston insert orifices 37 and theventing relief valve 31 the flow is inadequate during a rapid pressurebuildup to prevent a positive pressure differential on the front side ofthe piston. The pressure differential will cause rearward movement ofthe piston 33 as the transient fluid rearward of the piston is vented bythe pilot controlled relief valve. The piston and its integral pistonrod 21 will move rearwardly the rear end of the latter advancing pastthe presently stationary control rod 54. Further movement will result inthe control rod spring 55 being contacted and partially compressed tocause the link 60 and pilot valve lever to be rearwardly biased thusincreasing the load of lever 63 on the pilot stem 53.

The pressure at which the relief valve will unload is thus increasedover its manually preset (adjusting screw 68) value.

Under a given condition the size of the orifices 37 in the pistoninserts will determine how rapidly the piston willmove rearwardly to thefull rear position. With piston 33 gradually moved to the full rearposition the pilot 53 is gradually biased to a closed condition thusterminating the operation of relief valve 31. In this extreme case thesystem and particularly the pressure line 19 is unloaded by a systemmain relief valve (not shown) which maybe set at a relieving pressurevalue to protect the system. i

The desirable result achieved resides in the fact that line pressurebuildup will be relatively gradual since a portion of the pumps outputwill be bled off during elevating line pressures. During sustainedperiods of elevated line pressure the bleeding off of the pumps outputis gradually restricted to permit a slow building of system pressure.The slow increase in line pressure will either result in powering of thedigger teeth to overcome the obstruction or system pressure building toa main relief valve (not shown) dumping pressure which will unload theentire system.

Upon the obstruction being overcome and the resumption of operatingpressure in line 19 and chamber A the relief valve 31 will close. Thespring 43 will reposition its piston 33 forwardly with fluid filling viaorifices 37 the chamber area behind the piston.

The invention having now been set forth, what l claim and desire tosecure under a Letters Patent is:

lclaim:

1. Variable relief means for incorporation into a hydraulic drive systemintermediate a source of hydraulic pressure and a poweredinstrumentality the latter being subject during operation to varyingloads causing elevated fluid pressure in the system, said relief meanscomprising,

a cylinder defining a chamber in fluid communication at its forward endwith a pressure line through which pressurized fluid is directed to theinstrumentality during normal operation,

a piston-piston rod assembly within said cylinder with said pistondisposed adjacent the forward end of the cylinder during normaloperating pressures in said pressure line,

orifice means permitting a flow of fluid to the chamber area rearward ofthe piston whereby during normal instumentality operation the chamber ischarged with fluid under uniform pressure,

a relief valve in fluid communication with the opposite end of thecylinder defined chamber and venting at a predetermined pressure value,

a pilot valve in operating communication with said relief valve andincluding adjustable means permitting opening of said relief valve atpredetermined pressure values,

means linking said piston-piston rod assembly and the adjustable meansof said pilot valve for translating rearward piston assembly movementinto the progressive loading of said adjustable means, and

said relief valve upon an elevated pressure value being reached adaptedto vent the chamber and primarily that chamber area rearward of thepiston whereby continued elevated pressure in the chamber area forwardof the piston will cause rearward movement of the latter toprogressively load via the linking means the adjustable means of thepilot valve thereby progressively increasing the pressure value at whichthe relief valve will vent to achieve relief valve closing in a gradualmanner.

2. The variable relief means as claimed in claim 1 wherein said orificemeans is housed within said piston in a manner to permit substitution ofother orifice means to permit varying the flow rate to the area rearwardof the piston and correspondingly the sensitivity of the piston todifferential pressures.

3. The variable relief means as claim in claim 1 wherein saidpiston-piston rod assembly is forwardly biased by a helicalspringinternall disposed in said cylinder.

4. he varra le relief means as claimed in claim 1 wherein saidadjustable means in said pilot valve includes a stem and said linkingmeans is operable to progressive load the stem for progressivelyincreasing the pressure value at which said relief valve will vent.

5. The variable relief means as claimed in claim 4 wherein said linkingmeans includes screw means for presetting the initial loading of thepilot valve stem.

6. The variable relief means as claim in claim 1 wherein said linkingmeans includes a resilient member contactible by the piston rod uponrearward movement thereof.

7. The variable relief means as claimed in claim 6 wherein said linkingmeans further includes a rod positionably mounting said resilient memberfor contact of the member by the piston rod at selected points ofrearward travel of the piston rod.

8. The variable relief means as claimed in claim 7 wherein said pistonrod and said rod are in telescopic relationship.

